Code division multiple access (CDMA) techniques have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another. Many communication systems utilizing code division multiple access techniques comply with the IS-95 standard, adopted by the Telecommunication Industry Association (TIA). Under the IS-95 standard, a communication system substantially eliminates co-channel interference and improves the bit energy-to-noise density ratio, Eb/No, on the forward link from a base station or cell site to a mobile receiver unit by modulating the information signals with Walsh orthogonal function sequences. To produce corresponding orthogonal information signals, these CDMA systems require that the forward link information signals be transmitted in a synchronized manner. A more detailed discussion of the IS-95 standard is provided in “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” Telecommunication Industry Association Doc. No. TIA/EIA/IS-95 (1993), incorporated by reference herein.
Under the IS-95 standard, the forward link generally consists of a number of administration channels, with the remaining channels being utilized for voice and data signals. One of the administration channels is a pilot channel, which is an unmodulated, direct-sequence spread spectrum signal transmitted continuously by each CDMA base station. The pilot channel is modulated by a pilot PN sequence, consisting of a pair of modified maximal length PN sequences (in-phase and quadrature) with a period of 215 chips. Different base stations are identified by different pilot PN sequence offsets in units of 64 PN chips of a pilot, relative to the zero offset pilot PN sequence.
The pilot channel allows a mobile station to acquire the timing of the Forward CDMA Channel after a phone is initially turned on (“initial pilot detection”). During the voice and data modes, the pilot channel enhances the overall signal quality by providing a phase reference for coherent demodulation and a means for signal strength comparisons between base stations for determining when to handoff a call between two cells (“continuous pilot detection”). In addition, CDMA phones have a slotted mode for power saving, during which time the phone only operates at given slots to keep the connection alive while going into a sleep mode for the rest of the time. In order to maintain the timing of the Forward CDMA Channel each time the phone returns to an operative mode at given slots, the pilot detector must research all the signal paths before the phone wakes up at its slot (“slotted pilot detection”).
The pilot detector must be able to locate new signal paths with the lowest possible false alarm probability within the shortest possible time. Such requirements, however, are mutually exclusive and trade-offs are required. The trade-offs are largely limited, however, by the complicated and time-intensive noncoherent combining/noncoherent detection (NCND) scheme that is employed in current system designs. In addition, the noncoherent combining severely degrades the detection performance. For a detailed discussion of noncoherent combining/noncoherent detection, see, for example, Andrew Viterbi, CDMA Principles of Spread Spectrum Communication (Addison Wesley 1995), incorporated by reference herein.